stimuli calling forth activity, but physiologists can also orient at will the lower forms of life, both plants and animals, by means of light, heat, electricity, acids, gravity, hard surfaces, etc. This capacity of response is called in its various manifestations, heliotropism, galvanotropism, chemotropism, geotropism, and stereotropism. Different plants and animals respond in various ways to these mechanical stimuli, the same stimulus in some cases attracting one form and repelling another, and the same organism responding in different ways to different degrees of intensity of the same stimulus, but there is no doubt that the control of the sentient life of the lower stadia of living matter falls directly under the fixed laws of physics and chemistry. All instincts of animals, their habits, sagacity, migrations, and reproductive life, find their explanation largely in these "tropisms." The views of certain recent writers, including Mr. Romanes, on the nature of animal intelligence were really obsolete when they left the press.
In the more lowly organized forms all stimuli are mandatory. Hunger, for example, calls out motion through changes produced by katabolism in the organism, whereby it is attracted chemotropically to food. In the higher forms higher nervous centers and the power of inhibition and choice are developed, along with special organs of sense and locomotion. What is important for our purpose is that, given the property of irritability, due to chemical constitution, which responds by movement to certain stimuli and exhibits through dissociation of atoms an energy which from being purely mechanical becomes more and more purposive, we have a starting point for the interpretation of the psychical energies of man. Differences in temperament in individuals and in races must be regarded as due to the same causes as positive and negative heliotropism or chemotropism in plants and animals, namely, chemical constitution. The temperament, character, or genius of a people predetermines within certain limits the spirit of its institutions; it is the direct exponent of the quality of racial irritability, and apparently more fundamental, persistent and inimitable than intellectual
